In conventional wood frame building construction, a wood sill plate is attached to the top face of the foundation by a threaded bolt or a mud sill anchor such as Simpson Manufacturing Co., Inc.'s, U.S. Pat. No. 3,889,441. The wood wall studs are attached to the wood sill by toe nailing. This standard construction is still used throughout most of the United States. In areas subject to earthquake activity such as California or tornado activity in the midwest and hurricanes in the southeastern states, building codes have been inacted to require some further anchoring of the wood frame structure to the concrete foundation. The early holdowns were merely metal angles and gusset members. They were inadequate because they were designed for holding two wood members which assumed means of attachment generally equal in respect to the resisting and loading forces.
The first generation of holdown devices which were designed to respond to the needs of connecting two very dissimilar materials such as wood and concrete was an angular channel design consisting of a relatively long body with means provided for two or three bolts intended to be attached to a portion of the wood frame such as studs and with a seat element welded into the end of the device. A single hole was formed in the seat for receiving an attachment means therethrough such as a bolt or anchor embedded in the concrete foundation or floor slab.
In 1965, the Simpson Company of San Leandro, Calif., in response to a growing need for holdowns to meet the more restrictive building codes, brought out a line of devices for connecting wood structures to concrete foundations and slabs for the purpose of resisting uplift forces generated by earthquakes. A typical holdown first designed and manufactured by Simpson and still sold today is illustrated in FIG. 10. This device consisted of a back formed with two or three bolt holes and triangular sides. A seat member was welded to the sides and back and was formed with a bolt hole for receipt of the bolt embedded in the concrete. The seat member was elevated above the wood sill so that the code requirement of a distance of seven (7) times the diameter of the bolt between the end of the stud and the center line of the first bolt in the device could be achieved. The Simpson Company did not file for patent protection on the device illustrated in FIG. 10 and as a consequence they were copied and became the industry standard.
The first variation in the holdown illustrated in FIG. 10 was made about a year later and is shown in FIG. 11. The former angled channel section became one-sided and the seat element was formed integrally with the side member. This design variation was practical for the lighter models and is still made and sold today. No patent action was taken on this variation, and it also became the standard of the industry.
In 1972, Gilb was granted U.S. Pat. No. D. 224,083 on a holdown illustrated in FIG. 12. This design incorporated a spacer element which automatically provided the "seven bolt diameter" standoff distance and provided gusset support in the seat.
The prior art holdowns had the following problems and shortcomings:
1. All of the prior art devices present a wasteful excess use of materials in respect to the load functions performed.
2. The triangular channel section holdown of FIG. 10 by its nature, when viewed as a flat plane prohibits any possibility of economical progressive die manufacture and requires relatively primitive hand, shear, punch, and bend operations in its fabrication.
3. The fitting and 100% welding of the seat insert of the holdown of FIG. 10 is costly and requires hand-fitted miscellaneous iron operations.
4. The basic design of the prior art holdown of FIG. 10 presents substantial field installation problems. The side supports enclose most of the bolt heads or nuts and a socket wrench is required for installation. As carpenters, who normally are responsible for installing the devices, normally only carry conventional end wrenches it is necessary for them to carry an additional tool in their tool boxes. In some union jurisdictions, carpenters are not even permitted to carry a socket wrench and it is necessary to call in another union to install the holdowns.
5. The most serious problem with the prior art devices is the fact that the nature of their design causes an angular moment reaction with respect to the imposed loads. Solving this design problem requires thicker gauges which contribute to the general heaviness of the device. As shown in FIG. 11, in the prior art the anchor bolt is attached to the seat at a horizontal distance from the back of the device. Since no part of the holdown structure is below the seat for resisting moment forces, and there is an eccentric loading on the device, the entire device tends to rotate about a pivot point where the seat intersects the stud. The result of this eccentric loading is an angular withdrawal of the bolts in the wood in respect to the relatively fixed resistance imposed by the bolt embedded in the concrete. Building codes require a design safety factor of 3, but no matter how massive the device, a characteristic mode of failure is the splitting and destruction of the stud by the downward and angular movement of the stud bolts. The intended shear value of the bolts in wood is not properly used.